1. Field of the Invention
This invention relates to oil and gas well drill stem test tools and, specifically, to an annulus pressure operated test tool for use in a subterranean well.
2. Description of the Prior Art
As oil and gas wells are drilled, the drilling fluid or "mud" serves several important functions. One of the purposes of the mud is to maintain a desired level of hydrostatic pressure upon the formation being drilled to contain formation fluids during drilling. This is accomplished by weighting the mud with various additives so that the resulting hydrostatic pressure of the mud at the formation depth equals or exceeds the formation pressure.
Drill stem test tools are used to test the production capability of the formation. This can be done by lowering a test string into the borehole to the formation depth, and by allowing the formation fluid to flow into the string. A lower pressure can be maintained in the interior of the test string as it is being lowered by keeping a valve closed near the lower end of the test string. Once the appropriate depth has been reached, a packer is set to seal off the surrounding well bore and isolate the formation which is desired to be tested.
The valve at the lower end of the pipe string is then opened, thereby allowing the formation fluid which is isolated from the well hydrostatic pressure to flow into the interior of the pipe string. Various drill stem tests are then conducted which are familiar to those skilled in the art. Such testing typically includes periods of time in which the formation fluid is allowed to flow through the test string, as well as periods of time in which the formation is "closed in." By taking pressure readings during the testing intervals, the production capabilities of the formation can be estimated. After the testing is completed, a circulation valve in the test string is opened to allow the formation fluid in the test string to be withdrawn.
It is often desirable, especially in off-shore locations, to reduce or eliminate test string movement during the drill stem testing operations. This is due to, for example, safety considerations in maintaining the blow-out preventors closed during the testing procedure. Because of the desirability of eliminating movement of the test string, annulus pressure operated devices have been developed. Despite the fact that such devices are operable by annulus pressure with a minimum of test string manipulation, the prior art devices have all suffered from various disadvantages.
U.S. Pat. No. 3,824,850, issued July 23, 1974, to Nutter, shows an early annulus operated valve. The internal valve within the Nutter device was of the sliding sleeve variety and presented in restriction in the internal tubing bore which was undesirable.
U.S. Pat. No. 3,856,085, issued Dec. 24, 1974, to Holden et al, shows an annulus operated valve which utilizes a "full opening" rotatable ball valve to open and close the internal bore of the test string. Although the full opening bore of the tool was an advantage over the Nutter device, the Holden device utilized a valve operating force derived from the action of a trapped inert gas against a piston. This required that the operating personnel on the drilling rig be exposed to high pressure gas when filling and draining the trapped gas system. Trapped gas systems also require that the proper gas operating pressure be calculated for the test depth prior to inserting the device into the borehole. As a result, any unforseen change in the pressure at the test depth could affect the reliability of the testing device.